1. Technical Field of the Invention
The present invention relates to an image display device which uses a lens such as a lenticular lens or a fly-eye lens and can display images respectively directed to plural viewpoints and to a portable terminal device using the image display device. More specifically, the present invention relates to an image display device which has an excellent display quality without any brightness reduction in the reflection display mode and to a portable terminal device using the image display device.
2. Description of the Related Art
Since the distant past, various investigations have been made on a display device capable of displaying a three-dimensional image. Regarding the binocular vision, the Greek mathematician, Euclid, stated in 280 B.C. that “the binocular vision is a visual perception which a person can be obtained, simultaneously watching with his own left and right eyes two different images which are obtained by looking at a single object from different directions” (see, for example, Non-patent literature 1; “Three-Dimensional Display” written by Chihiro Masuda, Sangyo Tosho, K.K.). That is, the three-dimensional image display device is required in its function to provide two images having a parallax different from each other for the left and right eyes.
In the past, a greater number of three-dimensional image display methods were studied in order to actually realize such a function. The methods can generally be classified into those requiring to use eyeglasses and those requiring not to use eyeglasses. The anaglyph method using the color difference, and the polarized eyeglasses method using the polarization pertain to the method requiring using eyeglasses. Because it is substantially difficult to avoid troublesome resulting from the usage of eyeglasses, the methods requiring un-using eyeglasses have been mostly studied in the recent years.
The lenticular lens method, and the parallax barrier method and the like pertain to the method requiring un-using eyeglasses. The lenticular lens method was invented in 1910 or so by Ives et al. The parallax barrier method was envisaged by Berthier in 1896, and actually demonstrated by Ives in 1903.
The lenticular lens method was invented in 1910 or so by Ives et al., as described in the non-patent literature 1. FIG. 1 is a perspective view of a lenticular lens 121, and FIG. 2 is a diagram showing the optical model in a three-dimensional display method, using the lenticular lens. As shown in FIG. 1, the lenticular 121 has a flat plane on one side, and a plurality of hog-backed convex portions (cylindrical lenses) 122 extending in a direction, i.e., parallel to each other in the longitudinal direction on the other side.
As shown in FIG. 2, the lenticular lens 121, a display panel 106 and a light source 108 are arranged in this order from the side of an observer. A display panel 106 is disposed in the focal plane of the lenticular lens 121. In the display panel 106, pixels 123 for displaying an image for the right eye 141 and pixels 124 for displaying an image for the left eye 142 are alternately arranged. In this case, a group of the pixels 123 and pixels 124 adjacent to each other pertains to each convex portion 122 of the lenticular lens 121. As a result, the light, which is emitted from the light source 108 and then passes through each pixel, is deflected into two directions toward the left and right eyes by the convex portions 122 of the lenticular lens 121, so that images different from each other can be detected with the left and right eyes, thereby enabling a three-dimensional image to be detected by the observer.
On the other hand, the parallax barrier method was envisaged by Berthier in 1896 and later demonstrated by Ives in 1903. FIG. 3 is an optical model in a three-dimensional image display method, using a parallax barrier. As shown in FIG. 3, the parallax barrier 105 is regarded as a barrier (light shield) having a number of fine stripe-Shaped openings, i.e., slits 105a. In this case, a display panel 106 is disposed in the vicinity of one side of the parallax barrier 105. In the display panel 106, pixels 123 for the right eye and pixels 124 for the left eye are arranged in a direction perpendicular to the longitudinal direction of the slit. In this case, a light source 108 is disposed in the vicinity of the other side of the parallax barrier 105, i.e., on the side opposite to the display panel 106.
The light, which is emitted from the light source 108, and then passes through the openings (slits 105a) of the parallax barrier 105 and further passes through the pixels 123 for the right eye, becomes a light flux 181. Similarly, the light, which emitted from the light source 108, and then passes through slits 105a, and further passes through the pixels 124 for the left eye, becomes a light flux 182. In this case, the position from which an observer is able to detect a three-dimensional image is determined by the positional relationship between the parallax barrier 105 and the pixels. In other words, it is necessary to position the right eye 141 of the observer within an area through which all the light fluxes 181 for the pixels 123 for the right eye pass, as well as to position the left eye 142 of the observer an area through which all the light fluxes 182 pass. This relationship can be realized in the case when the midpoint 143 between the observer's right eye 141 and left eye 142 is located within a rectangular three-dimensional visible range 107, as shown in FIG. 3. Among line segments extending in the arrangement direction of the pixels 123 for the right eye and pixels 124 for the left eye, the line segment passing the intersection 107a of diagonal lines perpendicular to each other in the three-dimensional visible range 107 is the longest line segment. Accordingly, when the midpoint 143 coincides with the intersection 107a, the maximum latitude is obtained in the deviation of the observer's position in the horizontal direction, so that such a condition provides the most favorable observing position. In the three-dimensional image display method, therefore, the distance between the intersection 107a and the display panel 106 is regarded as an optimal observation distance OD, and it is recommended for the observer to observe the display in this distance. A virtual flat plane, which is positioned at the optimal observation position OD from the display panel 106 in the three-dimensional visible range 107, is denoted by an optimal observation plane 107b. Under this condition, the light emitted from the pixels 124 for the right eye and the light emitted from the pixels 124 for the left eye arrive at the right eye 141 and the left eye 142 of the observer, respectively. AS a result, the observer is able to recognize an image displayed on the display panel 106 as a three-dimensional image.
In the parallax barrier method early envisaged, a parallax barrier is interposed between the pixels and the observer's eyes, and therefore provides an eyesore so that the visibility is reduced. However, in the recent development of the liquid crystal display device, it is possible to dispose a parallax barrier 105 on the rear side of a display panel 106, as shown in FIG. 3, so that the visibility is improved. At present, extensive studies are carried out on the three-dimensional image display device, using the parallax barrier method.
However, the parallax barrier method is used as a method in which undesirable light rays are “hidden” by the barrier, whereas the lenticular lens method is used as a method in which the proceeding direction of light is altered. As a result, the lenticular lens method provides an advantage that the brightness on the display screen is not principally reduced. In view of this fact, an extensive study is made on the application of the lenticular lens method to a portable device which particularly requiring an increase in the brightness of the display and a reduction in the electric power consumption. In conjunction with the above, a transmissive liquid crystal display device is used as a display panel in a three-dimensional image display device using a conventional lenticular lens.
Besides the three-dimensional image display device, a display for simultaneously displaying plural images has been developed as an image display device using a lenticular lens (see Japanese Patent Laid-Open Publication No. H06(1994)-332354 (FIG. 20) referred to as Patent literature 1). This display simultaneously displays images different front one another in the direction of observation under the same conditions using the image distribution capability of a lenticular lens. This single display device can provide a plurality of observers, positioned in different directions with respect to the display device, with images different from one another. The patent literature 1 describes that the use of this display device can reduce the required set-up space and the power rate as compared with a case of using ordinary single-image display devices by the number of images to be displayed.
Up to this time, studies have been made on a reflective two-dimensional image display device having a reflection plate used in a display panel. In the reflective display device, the light entering from the outside is reflected by a reflector positioned inside the display device. In this case, the reflected light is used as a light source for display, so that it is necessary to use neither backlight nor side light as the light source. A transmissive display device, on the other hand, requires either backlight or side light as a light source. Using a reflective display device in a display panel can therefore achieve low power consumption as compared with the case of using the transmissive display device. In this respect, recently, active attempts have been made to adapt the reflective display device to portable devices or so.
However, in the case when such a reflective display device is used, the flat plane of the reflection plate reflects the exterior light as if it is a mirror, and therefore the other light source, such as a fluorescent light is imaged thereon, so that quality of display is deteriorated. Moreover, as for an observer, only the light incident from a specific angle contributes to the display, thereby causing the efficiency in using the exterior light to be reduced.
In order to eliminate such a problem, a technique in which a reflection plate is provided with a number of surface projections was proposed, as described in Japanese Patent Laid-Open Publication No. H08(1996)-184846 (Patent literature 2). FIG. 4 shows an example of a reflection plate having such surface projections. In this case, an organic film is deposited onto the reflection plate 4, and such surface projections 41 are formed on the under layer of the reflection plate 4 by forming projections on the surface of the organic film. In accordance with the profile of the surface projections, the exterior light entering the reflection plate in a specific direction is reflected, and then diffused in various directions. In this case, the exterior light entering in various directions is also reflected toward an observer. AS a result, the exterior light proceeding in various directions can be effectively used in the display by preventing the light source from imaging thereon.
Thus, both the three-dimensional image display device using a lenticular lens and the reflective two-dimensional display device have already been known in the technical field concerned.
Although the three-dimensional image display device using such a lenticular lens and the reflective two-dimensional display device have an advantage that a greater amount of reduction in the electric power consumption is attained, there exists no three-dimensional image display device, which is produced by combining the above two members with each other.
In view of this fact, the present inventors extensively studied such a combination of a three-dimensional image display device using a lenticular lens and a reflective two-dimensional display device, which combination makes it possible to realize a three-dimensional image display obtained in the reflection display mode, along with reduced electric power consumption. The result obtained reveals the following new problem.
In the three-dimensional visible range, which has been designed so as to exhibit substantially uniform brightness, the brightness in the display is reduced in several areas, depending on the observing position. At a shifted position where the brightness decreases, dark areas are discerned in the display, and a pattern of dark lines is observed in some cases. As a result, quality of a three-dimensional image display is deteriorated by the unevenness in the brightness.
Before discussing this problem, firstly a three-dimensional image display device using both a conventional transmissive liquid crystal display panel and a lenticular lens is described. FIG. 5 is a perspective view of a dual eye type three-dimensional image display device. A cylindrical lens constituting a lenticular lens 3 is disposed in conformity to two pixels (pixel 51 for the left eye, pixel 52 for the right eye) in a display panel 2. As shown in FIG. 6, the light emitted from the pixel 51 for the left eye or the pixel 52 for right eye of the display panel is refracted by the lenticular lens 3, and travels toward an area EL or ER. If, therefore, an observer places his own left eye 61 on the area EL and his own right eye 62 on the area ER, an image for the left eye can be detected with the left eye 61 and an image for the right eye can be detected with the right eye 62, thereby enabling a three-dimensional image to be observed.
Moreover, the size of substantial components in the three-dimensional image display device using the lenticular lens will be described, with the aid of the optical model shown in FIG. 7. In this case, it is assumed that the distance between the display pixel and the center of the convex portion on the surface of the lenticular lens 3 is H and the refraction index of the lenticular lens 3 is n. The center of the convex portion on the surface of the lenticular lens 3 implies the apex of the lenticular lens 3. One side of the lenticular lens 3 is a flat plane, and on the other side is convex cylindrical lenses each having a convex portion, that is, a number of hog-backed convex portions 31 extending in a direction are disposed. Furthermore, it is assumed that the focal distance of the lenticular lens 3 is f and the lens pitch is L. The pixels of the display panel 2 are disposed in a pair of a pixel 51 for the left eye and a pixel 52 for the right eye, where the pitch of the pixels is P. A pair of two pixels, i.e., the pixel 51 for left eye and the pixel 52 for the right eye pertains to a single convex portion 31. In this case, it is assumed that the distance between the lenticular lens 3 and the observer is OD, and that the expanded projection width of a pixel at the distance DC, i.e., the width of each projected image of the pixel 51 for the left eye or the pixel 52 for the right eye on a virtual plane which is parallel to the lens and is away therefrom by the distance OD is e. Moreover, it is assumed that the distance between the center of the convex portion 31 located at the center of the lenticular lens 3 and the center of the convex portion 31 located at the lateral end of the lenticular lens 3 is WL, and the distance between the center of the pair of the pixel 51 for the left eye and the pixel 52 for the right eye disposed at the center of the display panel 2 and the center of the pair of the pixels located at the lateral end of the display panel 2 is WP. Furthermore, the incident angle and exit angle of light in the convex portion 31 located at the center of the lenticular lens 3 are denoted by α an β, respectively, and the incident angle and exit angle of light in the convex portion 31 located in the lateral end of the lenticular lens 3 are denoted by γ and δ, respectively. The difference between the distance WL and the distance WP is expressed as C and the number of pixels within the area of the distance WP is expressed as 2 m.
Normally, the lenticular lens is designed in conformity to the display panel in most cases, so that the pitch P is regarded as a constant value. The refraction index n is determined by selecting the material of the lenticular lens, whereas the distance OD between the lens and the observer and the expanded projection width e of the pixel at the observation distance OD are specified respectively in predetermined values. Using these parameters, the distance H between the lens surface and the pixel and the lens pitch L are determined. From Snell's laws of refraction, the following expressions 1 to 6 hold, and from the geometrical relationship, the following expressions 7 to 9 hold:n·sin α=sin β  (Expression 1)OD·tan β=e  (Expression 2)H·tan α=P  (Expression 3)N·sin γ=sin δ  (Expression 4)H·tan γ=C  (Expression 5)OD·tan δ=WL  (Expression 6)WP−WL=C  (Expression 7)WP=2mP  (Expression 8)WL=mL  (Expression 9)
From expressions 1, 2 and 3, the following expressions 10, 11 and 12 can be derived:β=arc tan(e/OD)  (Expression 10)α=arc tan(1/n·sin β)  (Expression 11)H=P/tan α  (Expression 12)
From expressions 6 and 9, the following expression 13 can be derived, and from expressions 7 to 9, the following expression 14 can be derived. Moreover, from expression 5, the following expression 15 can be derived:δ=arc tan(mL/OD)  (Expression 13)C=2mP−mL  (Expression 14)δ=arc tan(C/H)  (Expression 15)
In this case, generally, the distance H between the pixel and the center of the convex portion on the surface of the lenticular lens is set equal to the focal distance f, as indicated in the following expression 16, so that the curvature radius r of the lens can be determined by the following expression 17:f=H  (Expression 16)r=H·(n−1)/n  (Expression 17)
Using a light ray tracking simulator which is obtainable on the market, computer simulations for a three-dimensional image display device were carried out on the basis of the above deign concept. FIG. 8 is an optical model of the three-dimensional image display used in the simulation. In this example, it is assumed that a display panel having a pixel pitch P of 0.24 mm is used and polymethylmethacrylate (PMMA) having a refraction index n of 1.49 is used for the material of the lenticular lens 3 under the conditions: the distance OD between the lens and the observer is 280 mm; the expanded projection width of the pixel at the distance OD is 65 mm, and m is 60. Then, it can be recognized that the distance H between the lens surface and the pixel is 1.57 mm; the focal distance f of the lens is 1.57 mm; the lens pitch L is 0.4782 mm and the curvature radius r of the lens is 0.5161 mm. From these parameters, it follows that the light reception surface 18 is located at a position away from the lens surface by 280 mm. Moreover, as a pixel pitch P is equal to 0.24 mm, the width of the pixel is 0.24 mm. A light emission area 17 was disposed in the center portion of the pixel. The width of the light emission area 17 was set to 0.186. Therefore, a non-display area having a width of 0.027 mm is disposed on either side of the light emission area 17. The light emitted from the light emission area 17 was diffused light. The non-display area is equivalent to a light shield section, which is used for preventing the color mixing in the display device as well as for transmitting display signals to the pixels. Moreover, in order to simplify the simulation, the calculation was made for only a pixel for the right eye, which is located in the vicinity of the center of the display panel.
FIG. 9 is a graph showing the result of a simulation, where the abscissa means the observing position on an observation plane which is away from the lens surface by a distance OD=280 mm and the ordinate means the illuminance at the observing position. It is found that the illuminance is high in an observing position range from −60 mm to 0 mm and the magnitude of illuminance is approximately constant within the range. That is, if the right eye is positioned within such a range, the right eye receives the light having a sufficient intensity, but the left eye can hardly receive light. This implies that, when an image for the left eye is displayed on a pixel for the left eye and when an image for the right eye is displayed on a pixel for the right eye in an actual three-dimensional image display device, the left eye receives the image for the left eye and the right eye receives the image for the right eye along with a sufficient image separation, thereby enabling a three-dimensional image to be detected by an observer.
Next, a computer simulation was carried out as for a reflective three-dimensional image display device wherein light emitting areas for pixels are disposed in a reflection plate. FIG. 10 is a diagram showing the optical model used in the simulation. Surface projections 41 are disposed only in a part of the reflection plate 4. This is because the difference between the surface projections and the flat portion is visualized. In a concrete way, projections having an inclination angle of 30° and a height of 2 μm are arranged with a pitch of 10 μm in three lines with respect to the center of the reflection plate. A light source 19 is disposed at a position apart from the lens surface by 1 mm such that the horizontal size of the light source covers all the lenses. The light from the light source 19 is diffused light. FIG. 11 is a graph showing the result of simulation. It is recognized that a decrease in the illuminance takes place at an observing position of −30 mm. That is, the display is observed in a reduced brightness, when observing at this position.
In this case, the simulation was carried out, as for only one pixel. However, it is generally that the surface projections are randomly distributed over all the display pixels. Hence, the brightness is detected in a varied state for respective pixels in the display device, and therefore a three-dimensional image, on which an image having a spatially varied brightness is superimposed, is observed, thereby causing quality of the three-dimensional image display to be deteriorated. Such a problem generally occurs not only in the three-dimensional image display device but also in the aforementioned display of simultaneously displaying plural images with respect to plural viewpoints.